The present invention relates generally to office chairs, and more particularly, to an apparatus for adjusting the position of a backrest relative to the seat of an office chair.
Many office chairs have backrests which can be adjusted in height or angle relative to a seat. It is also desirable to adjust both the height and the angle of a backrest so that the position of the backrest corresponds to the natural position of a user's back when the user is in a desired forward or reclined position. When a user reclines or leans forward, the torso generally pivots about an axis through the hip joints of the user. Thus, it is desirable to provide a backrest with a path of movement corresponding to the pivotal movement of the user's back.
Chairs having a mechanism for adjusting both the height and angle of a backrest are disclosed in U.S. Pat. Nos. 1,927,491 to Gabb and 3,351,379 to Street. The patent to Gabb discloses a back rest slidably accommodated in a two-part clamp carried by a link pivotally connected to the seat. The clamp is operable by means of a hand screw member which maintains support both with respect to its vertical and angular adjustment by cooperation with an arcuate-shaped fixed abutment associated with the seat frame.
The patent to Street discloses a similar mechanism including an integral portion of a seat frame provided with inner and outer cylindrically convex regions complementary to concave faces of a block. To adjust both the angle and height of the backrest assembly in relation to the seat frame assembly, a hand piece is rotated to allow disengagement of the complementary serrated parts and permit angular movement together of the block and stem of a backrest in relation to a seat frame.
Another type of backrest adjustment mechanism is disclosed in U.S. Pat. No. 910,357 to Case. The patent to Case discloses a seat frame provided with a back bracket which is formed with a T-shaped slide channel that extends longitudinally backward and upward along the arc of a circle. The lower end of a backrest shank is curved downward and forward to fit and slide longitudinally in the channel of the back bracket. To limit upward movement of the shank in the channel, a spring-latch is provided in the shank which is adapted to normally enter and operate in a small longitudinal groove in the channel. To lock the shank at any desired position, a wedge block is located in a recess in one side of the concave face of the shank. The block is adapted to be forced toward the adjoining wall of the slide channel by a screw having a handle.